Tailored geochemical additives to inhibit CaO-MgO-Al2O3-SiO2 (CMAS) melt formation in gas turbine engines

Abstract

Civil aircraft engines ingest significant quantities of mineral dusts during their operation in arid regions. These deposit on the engine components, melt at the elevated operating temperatures, and cause damage to the insulative Thermal Barrier Coatings (TBCs) that are critical to the durability of high temperature engine components. New melt-resistant TBCs may only mitigate damage effectively for specific deposit chemistries. We have investigated the use of tailored additives to change a deposit composition, raise its melting point and prevent melt formation. Only CaO-MgO-Al₂O₃-SiO₂ (CMAS) compositions were explored because deposits are typically simplified to this system. For our approach to work, it must be possible to reliably predict the composition, amount and melting temperatures of the deposited material and additive required to prevent melt formation at a given temperature. Experiments were performed between 1200 and 1400°C to investigate the chemistry and melting temperatures of a ‘deposit’ CMAS composition, and two ‘deposit + additives’ CMAS compositions produced by adding dolomite (CaMg[CO₃]₂) and/or periclase (MgO) to the ‘deposit’. We observed that enriching the starting material in CaO and MgO increased its melting temperature such that little to no melt would form on a high pressure turbine blade. Deviations of our liquidus temperatures from published liquidus diagrams show the need for further refinement of sections of the phase diagram. Greater understanding of the composition of airborne dusts around the world and their evolution inside aircraft engines is necessary before this approach can be used in practice.